Human skeletal muscles consist of a large number of motor units that are bundles of muscle cells acting in synchrony and excited by the same neuronal signal. The electrical excitation of the muscle causes the muscle to contract and carry out work. The electrical excitation in the muscle can be measured as a voltage between two electrodes in the tissue or on the skin. The operation of the muscle may be analyzed from the characteristics of this signal picked up by the electrodes. The properties of the voltage signal depend on the total activity of the muscle and on the relative activity of the different types of activated motor units within the given muscle.
The strength and endurance of muscles may be developed by planned exercise. To improve performance in sports, the muscles need to be exercised so that they get tired, but so that the muscles still recover from the exercise reasonably quickly. Also, there are many contemporary professions that tend to encourage a person to stay in the same position for a long time. This also causes the muscles to get tired.
With state of the art methods and devices for analyzing the operation of muscles, it has proven to be difficult to determine when a muscle is tired to an extent that it still recovers reasonably quickly, and when the muscle is tired to an extent that recovery takes significantly longer.
There is, therefore, a need for a solution that enables a more accurate and reliable analysis of muscle signals for determining the tiredness of the muscle for example for guiding a sports exercise.